macosx: Do not apply results of asynchronous image cache thumbnailFor... if the view's represented item changed

Since this series of methods from the image cache are async, it is possible for the target item in a view to change and receive a new thumbnail from the image cache before the old async request finished, therefore resulting in the old target item's thumbnail being set after the new one's

Signed-off-by: Claudio Cambra developer@claudiocambra.com